The invention relates to polymers of pivaloyloxyalkyl methacrylates containing structural units of the general formula I: ##STR2## where R.sup.1 is (CH.sub.2).sub.n.sbsb.1 --O (n.sub.1 =2 to 10) or (CH.sub.2 CH.sub.2 --O).sub.n.sbsb.2 (n.sub.2 =1 to 4); to the method of their preparation, as well as to the method for preparation of block copolymers consisting of a polymethacrylate block and a poly(pivaloyloxyalkyl methacrylate) block, by the polymerization initiated with anionic and free-radical initiators and to modification of the polymers and copolymers of pivaloyloxyalkyl methacrylates (including the block copolymers) resulting in the partial or total conversion of hydrophobic pivaloyloxyalkyl methacrylate units into hydrophilic hydroxyalkyl methacrylate polymers or copolymers.
Poly(glycol methacrylates) have a very broad application, e.g. in medicine, as materials for gel permeation chromatography an the like. Monomeric glycol methacrylate may be readily polymerized by the free-radical method. Anionic polymerization of glycol methacrylate is impossible because the hydrogen atom of the alcohol group of the monomer decomposes the anionic initiator by its acidity and deactivates the formed initiation centers. This is the reason why the possibility of blocking the alcohol group of the monomer by acylation was investigated. It can be assumed, from former studies by the inventors, that the successful polymerization of acylated glycol methacrylates may be realized with such acyl residues which contain only one or better no hydrogen atom in the .alpha.-position. However, it was proved that in the polymerization of methylmethacrylate using the organometal type of initiators, as n-butyllithium, and also using a group of less basic initiators as alkali metal tert-alkoxides (e.g. lithium tert-butoxide), the presence of an acyl residue with hydrogen atoms in the .alpha.-position, e.g., of alkyl acrylates, inhibit the polymerization reaction due to condensation reactions of acrylates with the aforesaid basic reagents.
By the procedures known until now, stereoregular poly(hydroxyalkyl methacrylates) and block copolymers containing both the lyophilic block(s) and the block(s) of hydrophilic hydroxyalkyl methacrylate structural units also could not be prepared.
According to the invention, the homopolymerization of pivaloyloxyalkyl methacrylates of the general formula II: ##STR3## where R.sup.1 has the above given meaning, may be carried out both by free radical initiators of the peroxy type and by anionic initiators. Anionic initiators used in the homopolymerization of the methacrylate type of monomers enable the advantageous preparation of polymers which substantially differ by their microstructure expressed as the tacticity degree. Thus, for example, lithium initiators form isotactic polymers in a hydrocarbon medium, while atactic to syndiotactic polymers are formed in a polar medium, e.g., in tetrahydrofuran. In conjunction with diverse tacticity, these polymers differ also in their mechanical properties, e.g., in the glass-transition temperature and the like.
The technique of preparing "living" polymers by anionic polymerization is not only the most advantageous one but virtually the only possible method for the preparation of block copolymers. It consists in polymerization of the first monomer by means of an anionic initiator up to the conversion of 100%, while the reactivity of the growing end of the polymeric chain is preserved at the same time and enables the second monomer added to be incorporated also.
Homopolymerization of pivaloyloxyalkyl methacrylates by the radical mechanism was effected by the use of free-radical polymerization initiators of the peroxy type, e.g. by dibenzoyl peroxide and isopropyl peroxycarbonate in a benzene solution and by a bulk process. The preparation of the pivaloyloxyalkyl methacrylate homopolymer in the presence of a crosslinking agent, e.g. glycol dimethacrylate, was also confirmed. Polymers with a network structure are formed during polymerization of pivaloyloxyalkyl methacrylates in the presence of a crosslinking agent, e.g. glycol dimethacrylate, which are insoluble in organic solvents and which only swell according to the amount of the crosslinking agent--glycol dimethacrylate--used.
Homopolymerization of pivaloyloxyalkyl methacrylate by an anionic mechanism was effected by the use of alkali metal tert-alkoxides, e.g. sodium tert-butoxide, or lithium organometallic compounds, e.g. n-butyllithium, as initiators. The polymerization by alkali metal tert-alkoxides proceeds at ambient temperature. The polymerization of pivaloyloxyalkyl methacrylates using n-butyllithium proceeds in bulk or in solution at the lowered temperature of -30.degree. to -70.degree. C. The anionic polymerization of pivaloyloxyalkyl methacrylates in the presence of glycol dimethacrylate was also confirmed. Polymers with a network structure are formed during polymerization in the presence of a crosslinking agent, e.g. glycol dimethacrylate, which are insoluble in organic solvents and only swell according to the amount of the crosslinking agent--glycol dimethacrylate--used.
If pivaloyloxyalkyl methacrylate contains a small amount of the crosslinking agent--glycol dimethacrylate, e.g. 1 to 2 percent, the suitably chosen concentrations of the monomer and intiator enable the preparation of a polymer which is soluble in organic solvents, as in benzene, toluene, chloroform, etc.
Block copolymers of methacrylates with pivaloyloxyalkyl methacrylates were prepared by the technique of anionic "living" polymers. The polymerization of the first monomer (methacrylate) was initiated by n-butyllithium in toluene solution at a temperature of -70.degree. C. to prevent potential side reactions of the polymeric-chain growing center with the ester group by a reaction of the Grignard type. This reaction proceeds at a temperature of e.g. +20.degree. C. to a considerable degree and leads to termination of the polymeric chains, which are then unable to bind further molecules of the monomer.
After the first monomer--methacrylate--had been consumed, the second monomer--pivaloyloxyalkyl methacrylate--was added and the polymerization was allowed to proceed to total conversion. The block copolymer obtained was then precipitated by methanol, reprecipitated and dried in vacuum at 45.degree. C. to constant weight. The methanol used contains a small amount of acetic acid to prevent alcoholysis.
The present method may be used for preparation of stereoregular homopolymers of pivaloyloxyalkyl methacrylates and block copolymers of these compounds with other methacrylates (methyl methacrylate, butyl methacrylate and the like). Poly(hydroxyalkyl methacrylates) or their block copolymers of corresponding tacticity may be obtained by a simple modification reaction of these polymers of various tacticity (isotactic, syndiotactic and atactic polymers). Until now, the preparation of a block copolymer which contained the blocks of hydrophobic and hydrophilic structural units has not been possible, because a monomer with hydrophilic groups reacted irreversibly with the anionic polymerization catalyst and interfered with the polymerization process.
The use of the monomers, which have their hydrophilic group protected in the form of a suitable ester, enables, besides the radical polymerization, also the anionic polymerization or copolymerization and also the block copolymerization. According to this invention, the protective groups in the prepared polymers are removed in part or totally giving rise to the hydrophilic hydroxyalkyl methacrylate structural units: ##STR4## where p is the polymerization degree and
n is an integer .gtoreq.2.
A feature of this process is the different cleavage rates of various types of ester bonds in one structural unit, namely, the substantial stability of the ester bond of the polymethacrylic skeleton and the relatively easy cleavability of the protective group ester bond.
The partial or total removal of the protective groups may be carried out in many ways, e.g. by hydrolysis, alcoholysis, aminolysis, etc., under conditions such that the difference between the higher reactivity of the ester bond of the protective group and the substantially lower reactivity of the ester bond of the polymeric skeleton remians preserved.
A suitable way can be, for example, alcoholysis catalyzed by the alkoxide which was used in the polymerization or was formed by the reaction of the anionic catalyst with added alcohol. The hydrolytic reaction can desirably be conducted at 0.degree.-100.degree. C.
The hydrolyzation of poly(pivaloyloxyethyl methacrylate) can for example be preferable carried out at room temperature.
After the desired degree of hydrolysis is achieved, the excess of the basis is neutralized, the aceton is evaporated and the polymer precipitated in water, in which the side products remain soluble.
The aminolysis in methanolic solution of ammonia practically does not run.
The reaction may be realized by the treatment with natriumamide in liquid amonia. The basis can be neutralized with ammonium chloride and after the evaporation of ammonia the polymer is soluted in boiling ethanol. The polymer is preferably precipitated in ether, in which pivalamide remains dissolved.
The reaction may be performed with soluble as well as with crosslinked (co)polymers, homopolymers and block copolymers.
Monomers containing more than one vinyl group may be advantageously used as the crosslinking agents, namely those selected from the group comprising ethylene glycol dimethacrylate or diacrylate, diethylene glycol dimethacrylate, glycol divinyl ether, divinyl adipate, divinyltoluene, trivinylbenzene, divinylbenzene, divinylnaphthalene, triallyl cyanurate, and the like.
The resulting modified polymers may be used in cases where the different affinity of the hydrophilic part of the macromolecule for polar molecules (e.g. water, biological fluids, etc.) and where the hydrophobic interaction of the nonpolar residue of the macromolecules plays a role, e.g. at phase interfaces, in the transport of polar or non-polar molecules through films and membranes formed by block copolymers--poly(hydroxyethyl methacrylate-b-methyl methacrylate), and the like.
The polymers according to the invention may be advantageously used, for example, as:
1. Synthetic leathers with hygienic properties - the properties may be varied by the ratio of methyl methacrylate and pivaloyloxyethyl methacrylate; and by the degree of hydrolysis.
2. Chromatographic materials, either in the nonhydrolyzed form or in the partially to totally hydrolyzed forms of protective groups.
3. Medical applications employing the basic nonmodified (co)polymer, which is then subjected to a suitable modification reaction (e.g. hydrolysis) to form the hydrophilic layer on the article surface.